The invention concerns a method of producing hollow profiles, in particular tubes, having a small external diameter and/or small wall thickness, made from a nickel titanium alloy by shaping a composite block.
Alloys having approximately the same amount of titanium and nickel atoms exhibit special effects leading to their designation as shape memory alloys. The effects are based on a thermoelastic martensitic phase change, i.e. a temperature-dependent change in the crystal structure: at high temperatures the alloy is austenitic; at low temperatures it is, however, martensitic. According to T. W. Duerig and H. R. Pelton (xe2x80x9cTI-NI Shape Memory Alloysxe2x80x9d, in: Materials Properties Handbook: Titanium Alloys, 1994, pages 1035-1048, ASM International 1994), shape memory alloys have two properties which should be distinguished from another. Alloys with a titanium content between 49.7 and 50.7 atom % have a thermal shape memory, also called shape memory, and alloys with a titanium content of 49.0 to 49.4 atom % have a mechanical shape memory, also called super-elasticity.
In addition to binary nickel titanium alloys, other alloys can also have these properties. A shape memory alloy can contain ternary components (e.g. iron, chrome or aluminium). The ratio between nickel and titanium and the presence of ternary additions strongly influence the markedness of the thermal and mechanical shape memory. Even slight concentration variations cause large changes in the material properties.
When using thermal shape memory for building components, an alloy of suitable composition is transformed, without diffusion, from an austenitic to a martensitic structure by cooling. Subsequent shaping of a component produced from this alloy can be reversed by thermal treatment thereof (heating to temperatures above a certain transition temperature). The original austenitic structure is thereby reproduced and the component adapts to its original shape. The transition temperature is usually the temperature at which the martensite is completely changed to an austenite. The transition temperature depends largely on the composition of the alloy and the loading of the component. Components having a thermal shape memory can cause movements and/or exert forces.
The mechanical shape memory effect occurs in a component made from a suitable alloy of austenitic structure when the construction unit is shaped within a certain temperature range. It is thereby energetically more favorable for the austenitic structure to change into a martensitic structure when loaded, wherein elastic expansions of up to 10 percent can be achieved.
Upon load relief, the structure returns to the austenitic phase. Components made from such an alloy can therefore store shaping energy.
Conventional alloys having the above-described properties are designated as Nickel Titanium, Titanium Nickel, Teenee, Memorite(copyright), Nitinol, Tinel(copyright), Flexon(copyright) and Shape-Memory-Alloys. These terms do not refer to one single alloy of a certain composition, but to a family of alloys having the described properties.
Many technical fields, e.g. medical technology and mechanics, are highly interested in use of components made from shape memory compositions due to the particular properties of nickel titanium alloys. In mechanical applications, they can e.g. be used for switching elements, actuating elements or valves. Shape memory alloys are also used to an increasing degree in medical technology, since components made from such alloys are biologically acceptable, fatigue-resistant and have also good flexibility as superelastic alloys.
Stents, catheters and endoscopic and laparoscopic instruments for minimum-invasive diagnosis and therapy are examples for use of nickel titanium alloys in medical technology, the intermediate product being a nickel titanium tube. Intermediate products in the form of tubes, in particular, of small external diameter, are also required for other applications.
Large-scale use of nickel titanium tubes and instruments is curtailed inter alia by the currently high price thereof which, in turn, results from the conventional methods for producing the tubular intermediate product.
Conventionally, nickel titanium tubes are produced by drilling forged bars. The tubes typically have an external diameter of between 12 and 25 mm. Due to the poor cutting property of nickel titanium alloys, the deep hole drilling method is difficult and results in short service life for the tools, long processing times and high production costs for the tubes. In addition, there is large material loss, in particular, when producing thin-walled tubes. The cuttings produced during drilling or turning on the lathe are waste material.
European patent document 0459909 describes manufacture of seamless tube from a corrosion-resistant alloy, consisting almost exclusively of titanium, using a tube extrusion method. In the method, a perforated press block is pressed through a gap between a press mandrel and a die using punch pressure. After subsequent shaping, the tubes generated in this manner serve e.g. for heating salt water in sea water desalination plants and as heat exchanger tubes in chemical production plants.
Due to the unfavorable shaping behavior of nickel titanium alloys, only tubes with a large external diameter (more than 40 mm) can be extruded economically using such a method. The extrusion of tubes having a smaller diameter is expensive since, due to the lack of cooling, it is not possible to achieve a sufficiently long tool service life in the temperature range dictated by the material. Moreover, the mandrels break off easily during extrusion, leading to a large amount of waste. The very large formation resistance of nickel titanium alloys at very high formation temperatures prevents production of small, thin tubes, since the press mandrel cannot withstand the high thermal and mechanical tensile loads which occur. According to prior art, therefore tubes having a large external diameter are initially pre-fabricated by tube extrusion and subsequently shaped into tubes of the desired small diameter using additional expending processing steps, e.g. drawing and rolling. Due to the advantages of nickel titanium alloys, in particular of shape memory alloys, these disadvantageous costs associated with the demanding production procedures are accepted in prior art.
WO 96/17698 discloses a method for lost core extruding of composite blocks without using a press mandrel. A block hollowed-out by drilling is filled with a steel core and both are extruded once together. The geometrical shape of the hollow extruded product depends on the geometrical shape of the extrusion die and of the core. The larger the core relative to the extrusion die, the thinner the wall. When thin-walled tubes are produced, this type of block preparation therefore results in considerable material waste, which is a substantial disadvantage with nickel titanium alloys. Moreover, in this method, the shaping process disadvantageously results in the metal core forming an intimate metallic connection with the nickel titanium material in the extruded product such that an additional processing step is required to remove the core material for obtaining a hollow extruded product, e.g. by drilling out and/or chemically removing the core material. Moreover, a desired small profile dimension cannot be achieved in all cases in a single extrusion of the composite block.
It is the underlying purpose of the invention to provide a method for the production of hollow profiles or tubes made from a nickel titanium alloy having a small external diameter and/or a small wall thickness in an inexpensive and effective manner. The hollow profiles or tubes may have any cross-sectional shape. The designation tube therefore refers to any profiled tube or hollow profile.
This object is achieved with a method for producing hollow profiles, made from a nickel titanium alloy, having small external diameters and/or small wall thickness through shaping of a composite block, wherein, in a first step, a composite block is formed comprising a solid core of a nickel titanium alloy, a first hollow block of a nickel titanium alloy surrounding the core, and a separating layer disposed between the first hollow block and the core. In a second step, the composite block is shaped by a formation method. In a third step, the first hollow block shaped into a first hollow profile and the shaped core are separated out from the shaped composite block.
The invention simplifies production effort and reduces nickel titanium waste by stabilizing the hollow block with a core during shaping, wherein the shaped core itself, with regard to its material as well as its shape, is suitable for further use. In accordance with the invention, the core is made from a nickel titanium alloy. The core, shaped into a solid full profile, can e.g. subsequently be used as wire or as an intermediate product in further processing steps. This reduces waste of the expensive initial material used in production.
In the method in accordance with the invention, an initial nickel titanium material is therefore divided into zones and the intermediate spaces are filled with a separating layer, e.g. a non-metal powder material, which does not bind to nickel titanium. The dimensions of the shaped products depend on the geometrical shape and the zone division in the composite block and on the formation method selected. The number and diameter of the individual zones can thereby be varied and depend on the shaped products desired.
The material forming the separating layer prevents contact among the individual zones before, during and after formation of the composite block to facilitate easy separation of the individual constituents of the shaped composite block from one another after shaping.
The shaping method can be varied. In a first advantageous embodiment, the composite block is extruded. The composite block is a heated up press block disposed in a block recepticle of a press and is extruded through a die opening by the pressure of a punch. The hollow block which is to be extruded into a tube is thereby stabilized during extrusion with a core inserted therein. The core can be removed after extrusion. In this respect, the method in accordance with the invention is analogous to one step of a multiple composite extrusion method. However, in contrast to the conventional, multiply repetitive composite extrusion, single extrusion may be sufficient. In addition, the obtained composite extrusion components are separated after extrusion to obtain a tube.
In contrast to tube extrusion, the method in accordance with the invention permits lower pressure extrusion operation which reduces wear of the pressing tools. Due to the small external diameter of the tube produced in accordance with the invention, optional subsequent shaping operations, e.g. cold-drawing, can be initiated with a smaller external diameter to save processing steps.
In a different advantageous embodiment of a shaping method, the composite block is formed by a warm-drawing, cold-drawing, rolling, round-hammering or pilger method.
In accordance with the invention, the tube is stabilized by the core during shaping. Within the scope of the invention, tubes can thereby be produced having a small external diameter and/or small wall thickness in an effective and inexpensive manner, despite the unfavorable shaping behavior of nickel titanium alloys having, in general, maximum shaping ratios during extrusion of 20:1.
The invention proposes various ways of producing the original composite block to be formed. In a first advantageous variant, the core is inserted into the first hollow block, in particular into a hollow profile, and preferably into a tube. Within the scope of the invention, a tube is a tube-shaped hollow block. Here, and also in other variants in accordance with the invention, it may be advantageous to form the composite block with one or more additional hollow blocks arranged around the first hollow block with a separating layer between each of the neighboring hollow blocks, wherein the composite block, comprising the several hollow blocks and the core, is shaped in the second step. This is particularly advantageous for producing several thin-walled hollow profiles in one formation step.
Moreover, it can be advantageous to form the composite block by inserting several hollow blocks, in particular hollow profiles, preferably tubes, into one another. In this respect, the previous and subsequent description equally applies to hollow blocks, hollow profiles, and tubes.
The hole in the hollow block for insertion of the core can be drilled or milled into a block or through a block. Since this always entails loss of material, even if the hole, except for the separating gap, is filled with a solid profile, an advantageous feature of the invention suggests forming the composite block, a hollow block or the core using cavity sink EDM or wire EDM in a solid nickel titanium block, a hollow nickel titanium block, or another nickel titanium workpiece.
It has been surprisingly shown within the scope of the invention, that electrical discharge machining, in particular using cavity sink EDM or wire EDM, facilitates advantageous processing of nickel titanium workpieces, especially when a part, in particular a solid core or a hollow profile, is taken out or separated from a block. Use of a tube-shaped electrode made from copper or from a copper alloy is preferred.
The method in accordance with the invention is directed towards the production of tubes made from a nickel titanium alloy, in particular a shape memory alloy as described above. The alloys used may be binary or may also contain ternary additives. The method serves preferably for producing tubes made from a nickel titanium alloy having super-elastic properties. The core, shaped into a solid full profile, also preferably has super-elastic properties.
With extrusion, the external diameter of the extruded composite block and therefore of the outer tube depends on the diameter of the opening in the die which cannot be arbitrarily small. The smaller the opening, the larger the pressure which must be used for extrusion and the shorter the service life of the pressing tools. In a preferred embodiment, the shaped composite block is inserted, before removal of the narrowed core, into a second hole fashioned in an additional hollow block made from a nickel titanium alloy to produce tubes of smaller external diameter. A multiple composite block is thereby made comprising the additional perforated hollow block and the first shaped composite block with the narrowed core, wherein the second composite block formed in this manner, comprises a separating layer between the first composite block, serving as a core, and the second composite block. A multiple composite bar is then extruded from the multiple composite block to thereby reduce the diameter of the additional perforated hollow block, the first hollow block and the core. This procedure can also be applied to composite blocks having several layers and to other shaping methods.
The shaped multiple composite block comprises a second tube formed from the second narrowed hollow block, the first additional narrowed tube and the additional narrowed core. After formation, the tubes are separated and the narrowed core removed. This two-step shaping process, e.g. extrusion, permits use of a larger die opening which is advantageous with regard to the required pressing power and the service life of the pressing tools. After separation of the tubes and removal of the core, two extruded tubes of different diameters are available for further processing.
If even smaller dimensions are desired, in particular for the innermost, smallest tube, formation or extrusion can be advantageously repeated once or a plurality of times until a predetermined external diameter for the smallest tube is achieved. Towards this end, the formed multiple composite block is inserted into another hole fashioned in an additional hollow block before the tubes are separated and the core removed, and the multiple composite block thereby formed is shaped to produce another tube. Insertion and shaping can be repeated, wherein an additional tube is made in each formation step. When this multiple step shaping or extrusion (multiple composite extrusion) is completed, all of the formed tubes are separated and the narrowed core is removed.
When the method in accordance with the invention is repeated once or a plurality of times, the once or repetitively narrowed core can also be removed along with the first innermost tube of the narrowed block (optionally together with one or more additional tubes neighboring the first tube), to subsequently insert a different core of nickel titanium or another material into the remaining block, which, in turn, can then be further reduced, i.e. either in its present form or after insertion into a further hollow block. Tubes with a uniform, small diameter can thereby be produced, wherein less tubes of larger diameter result.
Before formation, one hole is fashioned in each block or in each of the required hollow blocks, having e.g. a diameter between 10 mm and 60 mm, preferably between 20 mm and 40 mm. The hole is preferably eroded but can also be produced in the material in a different manner. In comparison to a blind hole, a through-hole has the advantage that, during formation, in particular extrusion, no solid piece, i.e. a section of the extruded bar without core, is generated which must initially be separated to produce a tube and which represents waste material.
In a preferred variant of the method, the composite block is shaped to a diameter which corresponds essentially to the diameter of the first hole of the core prior to formation. In this manner, the shaped composite block can be inserted into an additional hollow block having the same core diameter as the previous hollow block for forming a multiple composite block.
Advantageously, the multiple composite block can be shaped to a diameter corresponding essentially to the diameter of the composite block, serving as a core, before formation. In this way, the shaped multiple composite block can be inserted into a further hollow block having the same core diameter for forming a further multiple composite block.
The required diameter of the shaped composite block or multiple composite block is effected by appropriate dimensioning of the shaping tools. These method variants have the advantage that holes of uniform diameter can be formed in the required hollow blocks to reduce block preparation requirements.
In an advantageous manner, the external diameter of the first hollow block or tube after the first shaping step is less than 40 mm, preferably less than 25 mm. The smaller the external diameter of the produced tube or tubes, the larger the cost reduction for subsequent shaping operations. The wall thickness of a thin-walled tube is generally between 2% and 10% of the external diameter.